The present invention is directed to a method for structuring a lithography mask.
Because structure dimensions are becoming smaller and smaller for producing LSI semiconductor components, dimensionally true photolithographic transfer of mask structures onto radiation-sensitive resist layers is becoming more and more important. In the meantime, semiconductor components with structure line widths of 180 nm and less are being manufactured in a great volume for commercial employment, so that the demands made of the structuring process steps must meet the highest standards. In addition to the development of improved lithography techniques for the transfer of the mask structure, this also assumes the offering of finer and finer masks. The demands that are made of the structuring of the lithography masks are thus also becoming more strict.
Chrome masks are mainly employed both for proximity exposure as well as for contact exposure. These chrome masks are essentially composed of a silica glass substrate that is also light-transmissive in the UV range and have a thin, light-impermeable layer of black chrome applied onto the substrate. A back-reflection of incident light into the resist layer is minimized by the black chrome, which exhibits a low reflection coefficient.
Despite the widespread employment of wafer steppers with a demagnification factor of 4:1 in the commercial manufacture of semiconductor components, the structures to be imaged onto the masks have become so small in the meantime that the laser pencils that are currently still in widespread use must be increasingly replaced by electron beam printers, and, accordingly, the mask structures must be generated with electron beam lithography.
To that end, a resist layer is applied on the layer of black chrome on the mask blank and this is subsequently designationally exposed with an electron beam. As a result of the exposure, the resist is chemically modified so that the exposed regions comprise a different solubility in certain developer solutions compared to the unexposed regions. The solubility of the resist can be either raised or lowered by the exposure. When the solubility is raised, this is referred to as a positive resist, and the exposed regions are removed in the subsequent development of the resist. Analogously, a negative resist has the exposed regions of the resist remain on the mask blank. In an etching following the development of the resist, the chrome layer is removed in the regions that are no longer protected by the resist and, thus, the radiation transmissive regions of the mask are generated.
The employment of electron beam lithography is also essentially necessitated by new methods for the correction of what are referred to as “proximity effects”. What are understood by “proximity effects” are diffraction and interference effects of the mask structures that lie close to one another. These can lead to a noticeable deterioration of the obtainable dimensional accuracy. The proximity effects are all the more pronounced when the structures lie closer to one another. This, for example, results when structures that should actually have the same size are differently imaged into the resist layer dependent on their respective environment. This difference will appear especially clear between structures that are very densely arranged and structures that are largely isolated without neighboring structures.
In order to largely compensate for this difference, auxiliary structures—what are referred to as “scattering bars” or SRAF=sub-resolution assist features”—are usually employed, and are arranged in the proximity of the isolated structures. Accordingly, a structure that is actually isolated now has an environment that largely corresponds to the environment of the densely arranged structures, so that essentially the same imaging properties occur.
These auxiliary structures are thereby fashioned on the mask so that they themselves are not imaged into the resist layer, and they are respectively arranged parallel to the edges of the actual structures on the mask. Auxiliary structures of this type are disclosed, for example, in U.S. Pat. Nos. 5,242,770 and 5,821,014, whose disclosures are incorporated herein by reference thereto. These auxiliary structures are significantly smaller than the structure elements to be imaged and, as a result whereof stricter demands are made of the lithographic precision in the mask printing.
Another method for improving the photolithographic transfer of mask structures onto a substrate is the employment of what are referred to as phase masks, particularly “alternating phase masks” or ALT PSM for Alternating Phaseshift Mask. In contrast to standard chrome masks or COG for Chrome On Glass, two different transparent regions are generated in these masks. For example, this can occur in that, following the etching of the chrome layer on the mask blank, a part of the radiation-transmissive regions are provided with a phase boost by means of a designational etching compared to the respectively neighboring, radiation-transmissive regions in a second, following lithographic process so that a predefined phase difference is achieved between two respectively neighboring, radiation-transmissive regions. This phase difference will usually amount to 180°. An increase in the structure resolution of up to a factor of 2 compared to the traditional technique can be achieved by applying this technique given highly periodic, lattice-like structures.
The phase mask technique makes particular demands of the mask fabrication since additional layers are thereby applied or, respectively, the substrate must be eroded in defined fashion in order to achieve the desired interference effect. The manufacture of an “alternating phase mask” usually requires two separate lithography steps. The chrome layer of the mask is structured in the first lithography step. The charging of the mask observed given the employment of an electron beam printer does not yet represent a problem in this first step since the charge can be unproblematically dissipated by means of a suitable grounding of the continuous chrome layer of the mask blank.
In the second lithography step, the pre-structured chrome layer is coated anew with a resist, which must then likewise be inscribed, i.e. exposed. In this second step, the employment of an electron beam printer represents a problem because the chrome layer is interrupted by the structuring and, thus, a surface-wide elimination of the charge by grounding the chrome layer is no longer possible. As a result thereof, the pre-structured mask blank is negatively charged during the electron printing.
This negative charging influences the electron beam incident onto the mask blank during electron printing, because the electron beam is employed both for the writing, i.e. exposure, as well as for alignment monitoring. The interaction of the electron beam with the negative charge of the mask blank leads to an undesired deflection and spread of the electron beam, and will result in a disturbance of the alignment monitoring as well as distortions and writing errors when writing the second lithography level of the phase mask occur. This problem, which is also referred to as “charging”, currently complicates or, respectively, prevents the use of electron printers for the second lithography level in phase masks, so that optical mask printers are currently still utilized for this second step.
In order to be able to optimally utilize the phase mask technique, however, it is necessary that the phase-shifted radiation-transmissive regions of the mask are generated with at least the same precision as the structures generated by the first chrome etching. New methods with which this could be achieved should not, however, further lengthen the writing time of the mask, since the overall economic feasibility of the manufacturing process of the integrated circuit would be diminished as a result of the increased writing time. This must always be additionally taken into consideration in the development of new lithographic methods, and it is especially desirable for the writing time to be shortened further by means of the improved lithographic processes.
Previous approaches to solving the “charging” problem provide for the application of additional, electrically conductive lacquer structures on the pre-structured mask blank and are schematically shown in FIG. 1 as well as FIGS. 2A and 2B. The additional lacquer layers are usually composed of organic, electrically conductive components that are additionally selected so that they are water soluble. The water solubility of the electrically conductive layer is critical in order to thus avoid what are referred to as “intermixing” effects between the electrically conductive layer and the resist layer that is employed. Both the resist layer as well as the electrically conductive layer are applied by spin-on deposition onto the substrate. To this end, a solution of the respective lacquer components is applied onto the substrate during a fast rotation thereof, so that a thin surface film of the solution is formed on the substrate. Subsequently, the remaining solvent is eliminated from the spun-on layer. When a water-insoluble, electrically conductive layer is employed that is applied in the immediate proximity of a water soluble resist, then the solvent of the topmost applied layer in turn dissolves the layer lying thereunder that is already solid, so that no sharp boundary surfaces occur between the resist layer and the electrically conductive layer. This leads to an unsharpness in the later development of the resist.
In the approach shown in FIG. 1, an electrically conductive layer 16 is applied as an uppermost layer over the resist layer 14 for eliminating charges. FIG. 1 shows a substrate 10 of silica glass on which a pre-structured black chrome layer 12 has already been applied. A resist layer 14 was applied over the black chrome layer 12, and a water soluble, electrically conductive layer 16 is subsequently applied on the resist layer 14. The layer 16 additionally comprises an adequate transparency in order to allow a subsequent electron beam printing of the resist layer 14. This approach offers the advantage that the water soluble, electrically conductive layer 16 can be unproblematically eliminated in a wet-chemical step after the electron printing. However, the charge elimination is deficient given this layer sequence since the contact between the electrically conductive layer 16 and the substrate 10 or, respectively, the black chrome layer 12 is poor and a capacitor effect is generated due to the resist layer 14.
An alternative approach is shown in FIGS. 2A and 2B. In this approach, the electrically conductive layer 16 is applied directly onto the substrate 10 and the pre-structured black chrome layer 12, and the resist layer 14 is subsequently applied on the electrically conductive layer 16. After the electron printing of the resist layer , as indicated by arrows 100, this is wet-chemically developed.
Although the charge elimination given this layer arrangement is significantly improved compared to the version shown in FIG. 1, the problem does exist here that a pronounced undermining of the resist layer occurs in the development of the resist layer 14 due to an isotropic erosion of the water soluble, electrically conductive layer 16. As a result thereof, the dimensional precision with which the structure predefined by the electron printing is transferred into the substrate 10 or, respectively, the black chrome layer 12 is deteriorated in the subsequent etching step. This is shown in FIG. 2B. Given very small structures within the mask, the undermining of the resist can lead to these structures being completely washed away in the development. This is especially problematical given the structuring of masks with a plurality of auxiliary structures.